Dr Ken Ridgway of Purdue University and, previously, Chevron, spoke on a unique subduction zone in south-central Alaska. Here, in a situation similar to that at the Ontong Java Trench in the southwestern Pacific, a large oceanic plateau is subducting under the continent. Because the majority of oceanic crust is relatively thin and dense, it readily slides under continental crust, which is thicker and more buoyant. When a thick deposit of undersea lavas reaches a subduction zone, however, it causes problems, since such volcanic provinces are different from normal oceanic crust. Such a situation is called “flat slab subduction” because of the tendency of the subducting crust to go under the continent at a much shallower angle than normal.
In southern Alaska, the terrane, or chunk of crust, containing the thirty-kilometer-thick plateau is called the Yakutat terrane; it began subducting about thirty million years ago and is thicker in the direction of the ocean, such that it resembles a “door stop” being wedged under Alaska. The terrane is close to the surface as well, only fifteen kilometers deep beneath Mt McKinley. A very steep paleotransform fault (that is, a strike-slip fault that was active in the distant past but not any longer) separates the Yakutat from the Pacific plate. This part of the state boasts the highest coastal mountain range in the world, and this is due to some ten kilometers of sedimentary rock being scraped off of the subducting plateau onto the continent. Mount St. Elias, inland of the shore, represents the edge of the actual continent, or the “backstop” of the subduction zone. The scraped-off rock is highly faulted, with some 190 km of crustal shortening. Coal beds, representing former deltaic swamps, serve as the decollements, or sliding surfaces, for the faults. The presence of these coals indicates that, when they were deposited, the rock was at sea level, rather than far below it, with large deltaic systems pumping sediments onto the plateau. In order to determine the source of the deltaic sediments, detrital zircons and conglomeratic clasts were examined. The clasts give rock type and the zircons give rock age. By comparing the ages of the zircons with plutonic rocks (e.g. granitics) along the Pacific coast, the path along which the plateau moved can be ascertained. In this case, three plutons corresponding to the derived zircon ages of 58, 91, and 157 million years old were found to be far to the south, in the Coast Range of British Columbia, showing just how far the plateau has moved since that time.
Ridgway next described the younger rock record. The basalts of the plateau are from the paleocene, but during the eocene the plateau struck land, jamming up the subduction zone and stopping basalt extrusion until the miocene period, many millions of years later, when pillow basalts formed along faults in the area. At this same time, the Pacific plate abruptly changed its motion, suggesting the plateau’s collision with North America had something to do with the change. Likewise, volcanic eruptions in the overriding plate show a temporary shut-off from about thirty million years ago, the same time that the plateau hit. This is a known symptom of flat slab subduction, as the subducting plate is too thick to allow mantle material and heat to get through to the surface. Instead, the eastern side of the subducting slab caused “slab-edge” volcanism, with volcanoes forming at the margin in the overriding plate; the volcanism gets steadily younger to the north, showing the northwards migration of the plateau. These volcanoes are part of the modern-day Wrangell Mountains.
The collision of the thick plateau with the continent led to significant deformation of the continent as well as the slab. This can be seen in the progressively larger gaps in the rock record as one proceeds inland, where faults have removed sections of the stratigraphy. In the glacial sediments scraped off of the plateau, which consist of shelfal muds and turbidites (or underwater landslide deposits) with diamictites (a type of underwater glacial sediment) and dropstones (rocks that melted out of icebergs and fell to the seafloor), there are “growth structures”: angular cut-offs between successive layers of sediment which show continued movement along a fault as the angle steepens. Indeed, this area exhibits an abnormally thick record of deformation of the subducting plate, thanks to the scraped-off rock that makes up so much of the area. As for the continent, the backstop was uplifted about twenty to thirty million years ago as the Yakutat plateau plowed under it, with a fold and thrust belt (a heavily-faulted deformational zone) forming at tip of the uplifted area. In the miocene period, the western margin of the slab led to subsidence in the overriding plate, however, relative to the uplifted zone, just as a blanket will dip down at the edges of an object pushed under it; this is the modern-day Cook Inlet. Finally, a band of seismic activity, very diffuse and still active, extends far into the continent where the slab is going under, showing how it is still causing deformation today.
Ridgway ended his talk by pointing out the myriad effects the unique subduction regime in the area has had on topographic, seismic and volcanic activity, and petroleum exploration. Indeed, the first Alaskan oil field to be produced was discovered in the 1800s on the submarine fan deposits of the Yakutat, and the area is still being produced today.